Apparatus for determining amplitude of sound



w. F. ALDER June 16, 1936.

APPARATUS FOR DETERMINING AMPLITUDE OF SOUND Filed 001:. 22, 1931 3 Sheets-Sheet l Amen/7'01? M F4406)? firrakMs-vs.

I June 16, 1936. ALDER 2,043,983

APPARATUS FOR DETERMINING AMPLITUDE OF SOUND Filed Oct. 22, 1951 5 Sheets-Sheet 3 A TOP/VEYS.

reamed Ju e. 16, 1936 APPARATUSFOB DETERMINING AMPLITUDE OF SOUND William F. Alder, Altadena, calm, assign or, by mesne assignments, to Jenness A. Alder, Altadena, Calif.

Application October 22, 1931, Serial No. 570,486 4 8 Claims. (01. 73-51) My invention relates to an apparatus by which the amplitude, that is, the intensity or loudness of sounds may be accurately determinedand, in addition, that the sounds transmitted-through a material or reflected from such material may be measured. An object, therefore, of my invention is the provision of a .general apparatus in which there may be a source of sound and'the material to be tested may be arranged either for the transmission or the reflection of sound, the transmission of the sound through the material giving an indication of by such material, and the reflection of the sound giving a record of the intensity of sounds'reunder test. I

Another general object and feature of my invention is the provision of an instrument having a, suitable mounting in which material to be tested may be placed and also with a sound producing device in which the sounds may be directed either through or at an angle to the material. With this I utilize what might be termed a sound amplitude. meter, such being a device which gives an indication of the amplitude or loudness of the sound received by such instrument, such sound being either that transmitted through the material or reflected oil the face of such material. In this instrument I provide a protractor with suitable adjustments and clamps so that the material may be placed at a suitable angle so that the source of soundmay be arranged to project the sound at a desired angle incident to the material and the receiving sound amplitude meter. It may also be positioned to receive either the-transmitted a Another object and feature of my invention comprises an arrangement by using, preferably, a tunable resonator by which the source of so d may be accurately tuned to a or without its overtones and meter instrument is also resonator so that only the desired sounds are re- 4 ceived by'its resonator and used to give an indication in the amplitude determining instrument. Another object and feature of my invention as relates to the specific instrument for determining the amplitude of sound comprises a construction in which the condensation or compression waves 01' sound are utilized to build up aninternal air pressure, which air pressure actuates an indicating device v such asl a reflected of light or, apointer. For accurate work the reflected that the receiving the absorption of sound flected from various angles from the material or reflectedsound certain ,pitch with provided with 'a tuning light appears to be more desirable. In this pressure in the expansion chamber.

specific instrument I provide an expandible air chamber having, preferably, a flexible dia-' phragm on one side witha connection from such diaphragm to a tiltable inirror or to an oscillating arm. In orderto admit the compression or condensation portions of the sound wave and to prevent the exhaustion ofair by the rarefaction parts of the wave I provide a very delicate check valve as-an inlet to the air chamber. By this means I provide a construction by which the 10 compression or condensation parts of the waves may accumulatively build up an increased air This causes a deflection of the diaphragm and, hence, gives a swinging movement to the beam of light or moves a pointer arm, these latter devices being utilized tov indicate on a scale the pressure developed and, hence, they give a reading of the amplitude, that is, the intensity or loudness of the sound bywhich a test is being made.

Another characteristc of my amplitude detecting instrument and its use is that the check valve, if not subjected to sound, will allow the slow release of the trapped air in the expansion chamber whereby the condition of the device may return to normal, and in making tests I preferably sub-.- ject the instrument for a predetermined ,time to the sound waves and then have a predetermined period of rest, alternating these for a suflicient length of time to obtain an accurate average of the readings. I

A further detailed feature of my invention utilized principally for absorption and reflection 'of sound is a sound source, usually a siren electrically driven, with atunable resonator tube connected to the siren by which sounds of the desired pitch may be directed in the proper manner.

on or through the material under test, and in order to obtain the periods, of sound and quiet for allowing the sound amplitude meter to return to zero I may make and break the electric current, a simple way being to use a synchronous motor geared tomake and break a circuit at predetermined intervals. A comprehensive mount-. ing for a testing instrument may comprise a stand inwhich, on the vertical axis of the stand, there is mounted an adjustable holder for the material to'be tested. This holder is rotatable. i A plurality of radial arms extend from the stand,

one having a'source of sound thereon, such as an H electrically operated siren with or without the tunable resonating tube, and the other arm is provided with a sound amplitude meter, this preferably having a resonator tube. The axes of the resonators. are preferably ari'anged'on an of the particular frequency 2 7 axis with the vertical axis of the holder as .a center and these swing on a horizontal plane. The two arms may then be positioned tocause the reflection of the sound from the material to be tested at any desired angle, and in such test it is preferable to use a baflle between the source of sound and the receiving instrument so that only the reflected sound waves act on the amplitude meter. The arms may be positioned directly opposite one another for making tests of the absorption of sound.

Another manner of operating my testing instrument is to place the meter with the material holder and a resonator, if ,desired, in a. sound proof cabinet, the resonator extending outside of the cabinet to pick up and respond to sounds desired. In this case it is usual to turn the holder to secure the desired angle of reflection.

My sound amplitude indicator may also be used.

to pick up the complete sound by discarding the resonator and using a bell or a funnel in which to collect all sounds and noises, in which case the instrument may then be utilized to determine the amplitude of various sounds; such as ordinary noises. a

My invention is illustrated in connection with the accompanying drawings, in which:

Fig. 1 is a plan taken in the direction of the arrow l of Fig. 2 of a set up of the material holder on a stand, the sound source and the amplitude indicator also being connected to the stand;

Fig. 2m'ay be considered as a developed elevation taken on the right angled line 2-2 ofv Fig. 1 in the direction of the arrows;

Fig. 3 is a diagrammatic set up of the combined instrument as a testing device in a sound proof cabinet;

Fig. 4 is a diagrammatic plan of the amplitude indicating instrument set up for receiving all sounds;

Fig. 5 is a horizontal section through the casing of the amplitude indicator taken substantially on the line 5-5 of Fig. 6 with the interior parts shown in plan;

Fig. 6 is a. vertical section on the line '6-6 of Fig; 5 ,in the direction of the arrows omitting the distant interior elements;

Flg.-7 isan interior vertical sectionon the line 1-1 of Fig. 6 in the direction of the arrows;

Fig. 8 is ayertical section on the line 8-8 of Fig. 5 in the direction of the arrows;

Fig. 9 is an enlarged section on the line 9-9 of Fig. 6 in the direction of the arrows to illustrate principally the check valve;

Fig. 10 is a section on'the line ill-l0 of Fig. 9v with the inlet cover of the check valve removed;

Fig- 11 is a diagrammatic view of a suitable membrane for the check valve.

Referring first to the constructions of Figs. 1 and 2, giving a general set up of my instruments, a stand is indicated by the numeral H having suitable 'legs l2. Centrally positioned on this stand there is a table l3 having a material holder I42 This holder is in the form of a ring which may be 'rotated on its vertical axis. Such ring holds the material under test, which is in the form of a disk, by frictional engagement with the periphery of the disk. This table and holder are mounted to rotate on the axial center of the stand. A disk l5 surrounds the table and is rotatable on the stand. The joining edges of the table and the disk have graduations l6 giving the degrees of a circle. The outside of the disk also has similar graduations and stops the motor 24.

plan showing another gwalls 44 and the back '41 from which there l1. A screw l8 may be utilized to clamp the table and the material holderv on the stand; The table is also provided with a sound baiiie l9. vThis may be extended on opposite sides of the material under test and held in the 'holder, there being a sufllcient space at the center of the material for the reflection of sound a synchronous motor havinga commutator in which one section is provided with a conducting segment 28 engaging brushes 29. When the segment is in contact with the brushes the circuit is 2 closed and when out of contact the circuit is open Extended outwardly from the siren there is a resonator 30 comprising a tube with telescopic sections having graduations 3i and'having an adjustable means 32 con- 2 sisting of a pinion 33 meshing with a rack 34. This allows tuning of the resonator.

A second arm 35 extends from the stand an may swing in a horizontal plane and be clamped in any suitable position. This also has a pointer 3 36 passing over the degrees ll of a circle. An upright stud 31 supports the sound amplitude indicator. designated generally at 38. This is likewise shown as being provided with a resonator 39 which is adjustable. A suitable curved scale 40 with a combined sighting telescope 4| and light source is indicated, and I provide the light to be reflected by a mirror in the amplitude indicating instrument with a telescope or eye piece to view the reflected light. The combined sighting tele- 4 scope consists of a pair of closely positioned parallel tubes or a single tube with a partition, thus forming two closely adjacent tubes, in one of which there is a light source and the other has telescope lenses. Therefore, the projected light 4 from the light source and the light reflected back to the telescope follow practically the same paths Referring especially to the amplitude indicator: This comprises a housing designated generally at 42 (note Figs. 5 through 11). The housing has 5 a socket '43 at the bottom. It is preferably made with substantially cylindrical walls 44, a backing plate 45, and a removable cover plate 46. The plate 45 have an opening is a closed built out structure 5 48 having a window 49', preferably with curved glass. This is to allow viewing of the mirror and reflection of light from the mirror of the instrument. The cover plate is secured to the walls ofv 60 the casing by bolts 50.

Inside of the casing there is mounted a U- shaped casting 5|. This is indicated as having a pair of lugs 52 surfaced on their face and secured to the plate 46 by bolts 53. This castinghas a cross bar 54 and side arms 55. An upper bar 56 extends across between the side arms parallel to the bar 54 and outside of the side arms 55, and at the ends of the bar there are a pair of leaf springs .51. To these springs there is secured a wire 58 by studs 59. The spring may be pressed outwardly 7 by screws 60 threaded through lugs 61 on the bar 56. This is to give a tension to the wire carrying the mirror 62.

The bar 54 of the casting 5| has a small bore- 64 and a larger bore 64. In this larger bore 7 there is fitted a tube I having a head I to which is secured a disk 69 with a fiange 10. To this secured a thin diaphragm 1|,

flange there is I preferably made of metal, and attached by a 5 ring 12 having screws 13 therethrough. A pressure link 14 is attached to the center of the dia- I phragm and passes through an opening in the bar it andengages the mirror to tilt the same. The disk 69 form an air chamber 15.

In order to adjust the relative tension between the mirrorand the diaphragm, the tube 85 is threaded as indicated at 16 and is secured in a segmental threaded adjusting disk 11, this having a cutout notch 18. This disk is held in place by brackets 19. A rod 80 having a finger grip end 8| is attached by its inner end 82 to the disk 11. The wall 44 of the casing has an areaate slot 83, through which the arm 80 extends and allows a swinging movement, the partial turn being suiiicient to thread the tube 65 in or out relative to the supporting disk 11, and thus change the tension on the link 14. A lubricating duct 85 leads through the casting 5| so that no air can enter around the tube 85 and the bore in which it is fitted. A sound conveying tube 86 extends through an opening in the plate 46 and is securely fitted to the bar 54 portion of the casting 5|.

The delicate check valve 81 illustrated particularly in Fig. 9 comprises a disk 88 having a screw threaded flange 89, this structure forming an internal socket 90. In this there is fitted an internal disk 9! having a shoulder 92 and across this shoulder there is fitted a closure plate 93. This plate fits tightly against the inside surface 94 of the rim of the disk 9|. There is a relativelylarge opening 95 through the disk 88 and small openings 96 and 91 through the plates 93 and the disk 9|. The inside of the disk 9| has a slightly coned surface 98 with, radial grooves 99. The opening 96 is closed by a movable valve closure I00. This must be exceedingly delicate and I have found for this purpose abees wing is satisfactory, such being indicated in general in Fig. 11. The characteristic of the 'bees wing is that it has a very thin membrane Ill and on one surface of this membrane there are a series oi fine hairs I02. This membrane is attached to the plate 93 by a sealing medium, such as wax.

it, with the hair surfacev of the wing toward the plate 93. This wholeassembly is threaded in the threaded seat I04 formed in the casting II. On the tube 86 may be formed the resonator tube.

The action of the valve and the sound amplitude indicator with the air chamber is as follows: The sound is received through the resonator tube and, as above mentioned, this may be tuned to the desired frequency of sound, thus eliminating sounds which it is not desired to use. The

condensation or compression parts of the sound wave force the thin valve closure inwardly and allow the compressed air to enter through the 765 ports as and 91', building up an air pressure in the air chamber II. On the rarefaction or decompression part of the sound wave the delicate valve closure seats against the plate 33 and prevents decompression in the air chamber 15; that m is, it prevents the outward movement-of the air. The action 'is cumulative, each individual sound wave causing a building up of the air pressure in the air chamber 18 -by the compression or condensation part of the wave, the rarefaction 75 part of the wave having no function whatever and the diaphragm 1|, therefore, I

as regards the air pressure. The action continues until there is a pressure of air in the chamber 15 which becomes normal while the sound is being continued and shows no increase, or decrease, the showing being through the 5 movement of the link 14 and the mirror 62, which mirror may reflect light on a scale as above described. C

When the sound is discontinued and m sound waves are striking the thin membrane I00, the 10 hairs I02 on this membrane slightly space this from the closure disk- 93 and allow air to gradually flow out of the air chamber 15 around the membrane until the air pressure inside and the atmospheric air pressure outside are equalized. 15 This causesa return to zero of the reading by the reflected light. The object of having the sounds produced for a definite period of time and then having the silence is to obtain a number of readings in which the pressure is built up 20 to a maximum and retained at this maximum; for a suflicient length of time to know that no greater pressure may be obtained from that particularsound tested, whereby the average readings may be utilized to obtain what may be con- 25 sidered as the correct reading for the particular sound under test. It will be seen, therefore, that the membrane with the valve func tions to register only the energy of one half of the sound wave but this is suflicient to obtain 30 a reading of the sound energy and, therefore, by a proper scale reading the amplitude, that is, the intensity or loudness of the sounds. 1

In Fig. 3 I illustrate another set up for meas- In this case astand Hi5, has a 35 I06 with a material holder I01 thereon. A ring I08 on the stand ismarked with graduations to indicate the degrees of a circle. This stand has an arm on which there is a resonator M9 for receiving sounds and another arm on which is supported the sound amplitude indicator 0 having a tunable resonator III. A

back to the telescope. as having a circular The cabinet is illustrated end H6 with a slot H1 50 through which projects the resonator I09. This the receiving instrument. In some cases it is desirable to pick up all of the sounds. In Fig. 4 I show-a modification in whichfthe amplitude indicator designated Hills provided with a receiving bell I20 which may be of any suitable design and is adapted to pick 1 up all .of the sounds of whateveri'requency. This device, therefore, may be utilized to determine and indicate the amplitude of ordinary noises.

. movement of tude indicating instrument are v time and then allowed The scale on which the readings from the amphobtained may be graduated in any desired English system is used the graduations may indicate pounds of pressure per square inch in the sound expansion chamber or may be graduated to indicate toot pounds of work done in moving the diaphragm. Manifestly, for dealing with minute quantities, these will be in decimals. I find it advisable, however, to use the metric system, such as the centimeter-gram-second system, in which thescale may be graduated to indicate the force developed as dynes per square centimeter or the work done in ergs; or the scale may be arbitrarily graduated and used to cation of the amplitude of sound.

From the above description and drawings it will be seen that 'I have developed an instrument which is responsive to sound waves to indicate the positive pressure built up by the condensation or compression portions of the sound wave, eliminating the influence of the rarefaction or decompression portions of the sound wave. And further, the indicating instrument may be allowed to receive the sounds for a predetermined length of to recover the normal prestests may be used to the specific sound amplitude indicator, I have developed a comprehensive apparatus and procedure for developing and projecting sounds, and for obtaining accurate readings of the angles from which sounds are reflected from material under test to the receiving instrument, and also for determining the absorption qualities of the materials by rearranging the relation of the sound producing and the receiving instruments.

An important characteristic of my invention is that the sound waves from the sound emitter may be directed onto the face of the sample of material to be teste'dat any desired angle of incidence and the sounds reflected therefrom, which cause the resonator to respond, may be used to determine both the characteristics of the reflection of the sound from the surface and by comparison with the projected sound, the absorption. The absorption characteristics of also be determined by projecting the sound on one face of. the material imder test and placing the receiving resonator on the opposite side, this resonator then responding to the sounds'which pass through. the material.

Another important characteristic resides in the check valve formed of the extremely thin membrane I", this membrane being fitted across the openings and in contact with a flat surface. While the membrane is illustrated as being secured at one side of'the opening, it is obvious that it may be secured at opposite sides, if desired.

This membrane has'the function of operating so quickly that even for notes of high pitch it passes a small amount of air on the compression or condensation portions of the sound wave but closes on the decompression or rarefaction portions, thus accumulatively trapping minute quantitles of air which, therefore, accumulativeiy build up an increased air pressure in the expansible air chamber. A reading may then be obtained of the increased pressure in this sound chamberby the the diaphragm. forming a closure on one side of such chamber. It is manifest that various instruments may be used to determine the degree of eirpansion of the expansion air chamber and thus indicate the accumulatively built up air graduations and if the.

the material may.

give a relative indithe sound, producer is silent.

pressure in such chamber through the action of the compression or condensation portions of continuous sound waves.

An important feature of my invention resides in the sound wave rectifying valve having the thin membrane, such membrane being secured on the inside of the closed chamber, with the intake port through one side of this chamber. This valve functions with a longitudinally medium to transmit the compression or condensation phases of the vibration, whether these be rhythmic or non-rhythmic, such as noises, and either build up an increased pressure in the chamber or increase the volume of the chamber when the chamber is expansible. The vibrating membrane cannot vibrate outwardly on account of bearing against one surface of the chamber and, hence, does not respond to the decompression or rarefaction phases of the vibrations. Therefore, during these phases the inlet port to the chamber is maintained closed.

My invention in the sound wave rectifying valve is subject to many changes and to different types of uses. Some of these may be exemplified in my copending patent applications: Acoustic testing apparatus and method of operation, 9, 1932, Serial No. 597,828; Portable closed chamber determination of sound absorption, filed J uly 20, 1932, Serial No. 623,570; Gravimetric determination of sound amplitudes, filed July 20, 1932,

Serial No. 623,571. Various changes may be made in the details of construction without departing from the spirit or scope of the pended claims. I claim:

1. In a device as described, a stand having a holder for material to be tested, a first arm connected to the stand and having a sound producer thereon with means to direct sound toward the holder, 2. second arm connected to the stand and having a sound amplitude indicating device mounted thereon, said device having a sound receiver, said arms being adapted to rotate in a horizontal plane, and means to indicate the relative angle of the sound directing means and the sound receiving means relative to the holder.

2. In a. device as described, a holder for material to be tested, a sound producing means having a sound directing means connected thereto, a sound amplitude indicating device having sound receiving means connected thereto, means to position the sound directing and the sound receiving means at desired angles relative to the holder, 9., control for the sound producing means to produce and to cease producing sounds at desired time intervals, the sound amplitude indicator having a device to cumulatively allow building up of an air pressure during the condensation or compression portions of the sound wave and preventing lessening of this air pressure during the rarefaction or decompression portions of the sound wave, said device having means to allow equalizing of the air pressure 3. A sound amplitude indicating device comprising a structure having an expandible imperside sound receiving invention as defined by the apduring the time sound wave,

' sound being on the other sideof the baiiie, means membrane secured adjacent the flat surface, such flat surface being toward the sound to be received. said membrane admitting inflow of air during the compression 'or condensation portions of the sound wave, and preventing outflow of air during the rarefaction or decompression portions of the and thereby causing a building up of air pressure in the expandible chamber above that of the outside air, and means to allow escape of the trapped air between the membrane and the surface adjacent thereto.

4. A rectifying valve with a closed chamber structure and having'an inside surface with an inlet opening, a thin membrane secured and positioned to contact said surface and close said opening, said membrane having the characteristics of moving inwardly during the compression or condensation phases of a rhythmic or non-rhythmic vibration in a fluid medium, and successivelyv admitting portions of the fluid medium to'the chamber, and forming a closure for said opening. during the decompression or rarefactlon phasesv of the vibration and a releasing means on the said membrane engaging the said inside surface to equalize the pressure on its opposite sides when not subject to vibrations.

5. In a device as described, a holder for material to be tested, a sound baflle positioned in front of said holder and having an opening, a sound projecting instrument positioned to project sound on the material through the opening by an in-\ cident sound on one side of the baiiie, the reflected to receive the reflected sound, and means to indicate the amplitude of such received sound.

6. A sound amplitude indicating device having an expandible air chamber with a flat surface on the inlet side and with an inlet passage leading through such flatsurface, a check valve in said chamber formed of a thin membrane bearwhen unacted upon by sound waves.

ing against said flat surface and forming a closure for said passage, said membrane being movable slightly from said flat surface during the condensation or compression portions of the sound wave for entrance of air but being pressed 5 close to the flat surface d g the rarefaction or decompression portions of the sound wave to retain the trapped air and thereby building up. an air pressurein said air chamber above the exterior air pressure, and means operable by the expandible chamber to indicate the degree of expansion of such chamber, said membrane having a releasing means engaging said flat surface to equalize the pressure on its opposite sides when not, subjected to sound.

7. A sound rectifying valve having a structure with a flat surface and a sound inlet port through said structure and flat surface, a thin membrane formed of an insect wing secured to said structure and adapted to form a closure for said port, a chamber inside the port, said membrane flexing away from the flat surface during the condensation phases of sound-waves and forming a seal against said flat surface and closure of said port during the rarefaction phases of sound waves, the

area of said membrane contacting said flat surface being large compared with the area of said port, said membrane having hairs thereon en-' gaging said flat surface and breaking the seal 8. A device as described comprising a frame having a tiltable mirror mounted thereon, a hollow structure having a chamber with a movable diaphragm on one side and a threaded tube on the other side, a rigid connection between the diaphragm and the or, and a threaded adjustmg disc rotatable on the frame and threaded on c the tube with means to rotate said disc. 7

' WILLIAM F. 4o 

